This invention relates to thermosetting silicone resins that provide highly heat-resistant and optically transparent cured products with excellent hardness. This invention also relates to a method for curing these thermosetting silicone resins.
Silicone resins are polymers with a three-dimensional structure. They are obtained by the hydrolysis and condensation of organosilane compounds. A substantial amount of information exists on the preparation and properties of silicone resins, on methods for curing these resins and on the physical properties of their cured products (Silicone Handbook, edited by Kunio Itoh, Nikkan Kogyo Shinbunsha, page 468; Chemistry and Technology of Silicones, 2nd Edition, p. 409, Walter Noll, Academic Press, Inc. (London) Ltd., 1968).
Silicone resins generally include DT resins, polysilsesquioxanes, and in some cases, MQ resins. DT resins are prepared by the cohydrolysis of difunctional and trifunctional hydrolyzable silane compounds. Difunctional hydrolyzable silane compounds possess 2 hydrolyzable functional groups and produce the siloxane unit known as the D unit ((--O).sub.2 SiR.sub.2). Trifunctional hydrolyzable silane compounds possess 3 hydrolyzable functional groups and produce the siloxane unit known as the T unit ((--O).sub.3 SiR). Polysilsesquioxanes are produced by the hydrolysis of only trifunctional hydrolyzable silane compounds. MQ resins are produced by the cohydrolysis of monofunctional hydrolyzable silane compounds and tetrafunctional silane compounds. Monofunctional hydrolyzable silane compounds possess 1 hydrolyzable functional group and produce the siloxane unit known as the M unit (--OSiR.sub.3). Tetrafunctional hydrolyzable silane compounds such as silicon tetrachloride, in which all the functional groups are hydrolyzable, possess 4 hydrolyzable functional groups and produce the siloxane unit known as the Q unit (Si(O--).sub.4). In addition, many multicomponent resins, which can also be thought of as mixed systems of the preceding, are also known.
In cured form these silicone resins are used as heat-resistant coatings, protective coatings, electrical-insulating coatings, and so forth. These applications require that the uncured resin exhibit a good moldability and that the cured product exhibit heat resistance, hardness, etc.
Among the curing regimes available to silicone resins, the by-product-free curing reactions include (1) polymerization of silicone-bonded reactive organofunctional groups such as epoxy, methacryloxy, etc., and (2) hydrosilylation of an SiH and an Si-alkenyl (e.g., vinyl, allyl, etc). However, due to the low heat stability of the crosslink in each case, these methods are unsuitable for applications that require high heat resistance in the cured product.
When the particular application calls for high heat resistance, curing methods that produce a silox bond crosslink are often used. Many crosslinking reactions are known for this purpose. The following are frequently used:
1) Silanol group condensation EQU SiOH+SiOH.fwdarw.SiOSi+H.sub.2 O PA0 2) Alcohol elimination reaction between silanol and alkoxy groups EQU SiOR+SiOH.fwdarw.SiOSi+ROH PA0 3) Oxime elimination reaction EQU SiON.dbd.CR.sub.2 +SiOH.fwdarw.SiOSi+HON.dbd.CR.sub.2 PA0 4) Amide elimination reaction EQU SiN(R)C(.dbd.O)R.sub.1 +SiOH.fwdarw.SiOSi+HN(R)C(.dbd.O)R.sub.1 PA0 5) Acetic acid elimination reaction EQU SiOC(.dbd.O)CH.sub.3 +SiOH.fwdarw.SiOSi+CH.sub.3 COOH PA0 6) Acetone elimination reaction EQU SiOC(.dbd.CH.sub.2)CH.sub.3 +SiOH.fwdarw.SiOSi+(CH.sub.3).sub.2 C.dbd.O PA0 (a) a silicone resin of the formula (Ph.sub.2 SiO).sub.a (HSiO.sub.3/2).sub.b (R.sup.1 SiO.sub.3/2).sub.c and PA0 (b) at least 1 catalytic component selected from the group consisting of basic compounds, divalent and tetravalent tin compounds, palladium metal, platinum metal, palladium compounds, and platinum compounds. The catalytic component is present in the mixture at 0.01 to 10 weight % based on the silicone resin when it is a basic compound or a divalent or tetravalent tin compound. The catalytic component is present in the mixture at 0.00001 to 1 weight % based on the silicone resin when it is palladium metal, platinum metal, a palladium compound, or a platinum compound.
As is apparent, each of these reactions produce a by-product, e.g., water, alcohol, oxime, amide, acetic acid, and acetone. When organic compounds are eliminated during the cure, they can adversely affect the working and general environments.
When curing proceeds through formation of thermostable siloxane bonds by moisture-mediated curing reactions as commonly used for curing silicone resins (dehydration, alcohol elimination, oxime elimination, acetic acid elimination, acetone elimination, amide elimination, etc.), it is difficult to induce the formation of a thick, hard film. Moreover, a large weight loss occurs during cure. These factors cause substantial problems in terms of volume loss, warping, cracking, and so forth. Moreover, the fabrication of thick, hard monoliths is encumbered by the development of voids, cracks and dimensional instability in the cured material in association with vaporization of volatile by-products formed by the curing reaction.
It is also well known in silicon chemistry and the silicone industry that the hydroxyl group (e.g., of water, alcohol, silanol, etc.) will react with a hydrogen atom bonded directly to silicon to produce a hydrogen molecule and the silicon-oxygen bond, i.e., Si--O (refer to Chemistry and Technology of Silicones, 2nd Edition, p. 90; Organosilicon Compounds, p. 200, C. Eaborn, Butterworths Scientific Publications (London), 1960). Although the uncatalyzed reaction will run at elevated temperatures, it is widely known that this reaction will run more readily in the presence of a transition metal catalyst such as those of platinum, palladium, etc., a basic catalyst such as an alkali metal hydroxide, amine, etc., or a Lewis acid catalyst such as a tin compound, etc. Moreover, the use of crosslinking between Si--H and SiOH based on this reaction has been proposed as a room-temperature curing reaction for silicones (Chemistry and Technology of Silicones, p. 205, p. 397).
Resins containing large amounts of Q unit are used in applications where the hardness of the cured product is critical (e.g., in polysilsesquioxanes or DT resins). However, there is a tendency for the uncrosslinked resin to gel as the Q unit content increases. Moreover, the resin solubility, viscosity and processability also deteriorate. These problems can be addressed by using resins that contain the Q unit in the of a hydrolyzable group (such as an alkoxy group, etc.), but such resins suffer from large weight loss during cure.
Polysiloxanes containing diphenylsiloxane and hydrogensilsesquioxane as essential components have also been reported. For instance, Wu has reported cyclotrisiloxane and cyclotetrasiloxane composed of SiH(OSiA.sub.3)O and SiA.sub.2 O (A=aryl) and, similarly, polymer composed of SiH(OSiA.sub.3)O and SiA.sub.2 O (A=aryl) in U.S. Pat. Nos. 3,372,178 and 3,234,180. They are prepared by first synthesizing the 1,1,1-triaryl-2,2-dichlorodisiloxane by the reaction of triarylsilanol and trichlorosilane; then reacting this intermediate with diarylsilanediol to produce the cyclosiloxane; and subjecting this cyclosiloxane to ring-opening polymerization. In these siloxane compounds and polymers at least 1 triarylsiloxy group is bonded to the T unit (HSiO.sub.3/2) originating from trichlorosilane, so the practical degree of crosslinking at this T unit is a maximum of 3 bonds. The crosslink density is reduced still further by the presence of large amounts of the triarylsiloxy group, which is a bulky end group. It is noted that Wu did make reference to curing the polymers through hydrosilylation using the hydrogen atom on the T unit of this siloxane. Again, however, crosslinking based on the hydrosilylation reaction produces Si--C bond crosslinks, which precludes the preparation of a highly thermostable cured material.
Scholze et al. have received patents on siloxane coatings, porous polysiloxanes, and adsorptive polysiloxane resins for which the essential components--expressed in terms of precursors--are hydrolyzable silicic acid derivatives and silane derivatives having 1 to 3 hydrocarbon groups. The hydrolyzable silicic acid derivatives in these patents include Si--H-containing compounds in addition to silicate esters such as alkyl orthosilicate, silicon tetrachloride, and the like (U.S. Pat. Nos. 4,374,933, 4,243,692, and 4,238,590). These resins contain the Q unit, which originates from the highly hydrolyzable-functional silicic acid derivatives. When the siloxane-forming condensation reactions used to obtain these resins are not carried to completion, the resins are soluble in organic solvents and are fluid in resin form. However, one would anticipate curing problems such as large weight losses and the evolution of large amounts of volatiles.
Andrianov et al. obtained the bicyclopentasiloxane HSi(OSiPh.sub.2 O).sub.3 SiH (Ph=phenyl) by the reaction of trichlorosilane and diphenylsilanediol. Although this reaction was also reported to provide a polymer, the identity of this polymer was not made clear. In terms of its component elements the bicyclopentasiloxane reported by Andrianov et al. contains both diphenylsiloxane and hydrogensilsesquioxane. However, a curing reaction of this bicyclopentasiloxane by itself was not described (Andrianov, K., et al., Dokl. Akad. Nauk SSSR, 220(4-6) 1321 (1975)).
The present invention takes as its object a solution to the problems described above for the prior art through the introduction of a highly spin-coatable thermosetting resin that provides an optically transparent and highly heat-resistant cured product having an excellent hardness.
In order to accomplish the aforesaid object of the invention, the inventors carried out intensive and extensive investigations into increasing the crosslink density of the cured product without impairing the solubility, solution viscosity, spin-coatability, etc., of the uncrosslinked resin; suppressing the weight loss and volume shrinkage that accompanies curing; and obtaining a highly thermostable cured product.
Specifically, the inventors have discovered that a silicone resin containing both the diphenylsiloxane unit and hydrogensilsesquioxane unit can satisfy the above requirements.